1. Field of the Invention
The present invention is directed to the manufacture of masks used in the lithographic production of integrated circuits and, in particular, to the manufacture of embedded attenuated phase shifting masks (EAPSMs).
2. Description of Related Art
As an alternative to chromium on glass (COG) masks used in the lithographic production of integrated circuits, phase shifting masks (PSMs) have been employed in order to increase the resolution of the critical active area patterns projected. Such increased resolution enables smaller line widths to be exposed on the resist and consequently etched into or deposited on the wafer substrate. Some PSMs have used chromium as an opaque layer in conjunction with phase shifting image segments on the mask substrate, which correspond to areas of critical structures to be exposed with the PSMs.
Other PSMs for example, embedded attenuated phase shift masks (EAPSMs), utilize opaque layers of chromium to mask non-critical areas outside of the critical structure areas. As used herein, the term critical structures includes lines, contacts and other active regions to be exposed in a resist layer, subsequently developed, etched and/or deposited on the wafer to form an integrated circuit device or portion thereof. These phase shifting image segments typically impart a 180° phase shift of the polarization of the energy beam, e.g., visible or ultraviolet light. In manufacturing an EAPSM, a first exposure is typically made to create the phase shifting and opaque image segments on the substrate corresponding to areas of the critical structures, and a second exposure is made to selectively remove the opaque material in the area of critical structures but to leave the opaque material in the area surrounding the area of critical structures.
Typically, EAPSMs have been produced with costly laser or electron beam writing tools. While a typical COG mask may take 24 hours to fabricate, because of the requirement of utilizing two exposures, EAPSMs can take significantly longer to fabricate, up to 42 hours or more. This extended manufacturing time also ties up the use of the laser or electron beam writing tools. Additional costs are incurred in utilizing the laser and electron beam writing tools where the EAPSM is damaged or redesigned, or otherwise reworked, and must therefore be remanufactured. As a result EAPSMs while being extremely effective are also significantly more expensive to manufacture.